The present invention relates to improvements in rare-earth element type hydrogen absorbing alloys.
Hydrogen absorbing alloys are adapted to absorb hydrogen for storage and to desorb the hydrogen for use to utilize the desorbing pressure and the heat of endothermic reaction during the desorption as an energy source. The hydrogen desorbing pressure and the heat of endothermic reaction are utilized as drive sources for hydrogen compressors, actuators, cold generating heat pumps, etc. For this purpose, these alloys must be high in equilibrium hydrogen desorbing pressure around room temperature and great in capacity to absorb hydrogen and have outstanding durability to repeatedly absorb and desorb hydrogen.
The rare-earth element type hydrogen absorbing alloys already known for use include, for example, those disclosed in Unexamined Japanese Patent Publication SHO 60-197835 and represented by the general formula (Nd.sub.x R.sub.y)Ni.sub.5 wherein R is a rare-earth element other than neodymium or an alloy of the rare-earth elements, 0.4&lt;x&lt;1.0 and 0.0&lt;y&lt;0.6. These alloys are rare-earth element-Ni.sub.5 alloys having a specified amount of Nd incorporated therein so as to be given an increased capacity to absorb hydrogen and an elevated equilibrium hydrogen desorbing pressure around room temperature, whereas they have the drawback of being low in durability to repeatedly absorb and desorb hydrogen.
Unexamined Japanese Patent Publication SHO 60-70154 discloses other known examples of rare-earth metal-Ni.sub.5 alloys which are represented by the general formula LmNi.sub.a-x A.sub.x wherein Lm is one of rare-earth metals including 40 to 70% La, 0.1 to 2.0% Ce, Nd, Pr and Sm, A is a metal selected from the group consisting of Al, Mn, Fe and Cr, 4.8&lt;a&lt;5.5 and x is 0.01 to 2.0. These alloys are rare-earth metal-Ni alloys having Al, Mn or like metal incorporated therein and thereby are given improved flatness of plateau (stability of hydrogen dissociation equilibrium) and an increased capacity to absorb hydrogen at a relatively low equilibrium pressure (up to about 5 atm. around room temperature). However, these alloys are not suited for use as the energy sources mentioned above since they are not high in equilibrium hydrogen desorbing pressure around room temperature.
Although various other improvements in rare-earth element-Ni.sub.5 alloys have been proposed, hydrogen absorbing alloys have yet to be developed which possess all the three characteristics of a high equilibrium hydrogen desorbing pressure around room temperature, a great capacity to absorb hydrogen and high durability to repeatedly absorb and desorb hydrogen.
An object of the present invention is to provide a hydrogen absorbing alloy which has the three characteristics of a high equilibrium hydrogen desorbing pressure in the vicinity of room temperature, a great hydrogen absorbing capacity and high durability to repeatedly absorb and release hydrogen, and which is suitable as a drive source, for example, for refrigeration systems of the thermal drive type.